This invention relates to an apparatus for effecting the precise alignment of surfaces that are to be joined together. In particular, the invention relates to an optical system adapted for use in aligning an integrated circuit wafer or microchip with a patterned substrate.
The development of apparatuses for the precise alignment of surfaces that are to be joined together continues to be active. This is particularly the case in the area of microelectronic fabrication, where the patterns that are to be aligned are microscopic or nearly microscopic in scale. One such pattern may be on a surface of an integrated circuit wafer or an individual microchip and may comprise a variety of connectors, such as metallic wires or ribbons, or tiny bumps of solder, conductive epoxy or indium, that are formed thereupon. The complementary pattern typically would be on a surface of a substrate and would comprise a pattern of tiny pads or conductors to provide electrical connections to the microchip or to another patterned substrate.
In the commercial production of microelectronic devices, a die may be attached to a substrate during an automated process. In some approaches, the patterns of the die and substrate are aligned directly with each other, or reference marks for guiding alignment may be provided on the components or component carriers. Accurate alignment of components or reference marks may be facilitated using an optical system that provides enlarged images of the components superimposed one upon the other. A typical optical system is illustrated in FIG. 1. The optical system 1 includes an optical cube beam-splitter 2 and an image processor 3, which may contain components such as one or more cameras, a video monitor, and circuitry for signal processing and control of the assembly process. A die 4 having connector bumps 5a, 5b is suspended above the beam-splitter 2 by means of a die carrier 6. Suction applied through a port 8 holds the die 4 against the die carrier 6 during the assembly operations. A substrate 10 having electrical contacts 12, 14 is positioned beneath the beam-splitter 2 on a substrate carrier 16. Reference marks A, B are provided on the die 4 and reference marks Axe2x80x2, Bxe2x80x2 are provided on the substrate 10 to facilitate the accurate alignment, and hence, attachment, of the bumps 5a, 5b with the electrical contacts 12, 14, respectively.
Still referring to FIG. 1, the beam-splitter 2 is provided with two similar triangular optical prisms 18 and 20. The prism 18 has a transparent face 22, a 100% reflective mirror face 24 and a hypotenuse face 26, with the faces 22, 24 being perpendicular to each other. The prism 20 has a transparent face 28, a transparent face 30 and a hypotenuse face 32, with the faces 28, 30 being perpendicular to each other. Each prism 18, 20 also has a 45xc2x0 angle between the faces 24, 30 and the respective hypotenuse faces 26, 32. The prisms 18, 20 contact each other at their respective hypotenuse faces 26, 32, forming an interface 34 along the plane of contact. One or both of the hypotenuse faces 26, 32 is coated with a reflective material, such as a metal or a reflective dielectric material. Typically, these coatings provide the interface 34 with a reflectance of 50%, i.e, half of the light striking the interface 34 will be reflected and half will pass through the interface 34.
Light, provided by a source of illumination (e.g., a lamp) and striking the die 4, is reflected as an image of the die 4 comprising light beams 36a, 36b which pass through the face 28 of the prism 20 and strike the interface 34. A portion 38a, 38b of each light beam 36a, 36b is reflected by the interface 34 at a 90xc2x0 angle of rotation. The reflected portions 38a, 38b exit the prism 20 through the face 30 and is received by the image processor 3.
Light, provided by a source of illumination (e.g., a lamp) and striking the substrate 10, is reflected as an image of the substrate 10 comprising light beams 41a, 41b which pass through the face 22 of the prism 18 and strike the interface 34. A portion 43a, 43b of each light beam 41a, 41b is reflected by the interface 34 at a 90xc2x0 angle of reflection and is reflected back to the interface 34 by the mirror face 24, thereby being transmitted to the image processor 3.
The resulting image, viewed at the face 30 of the prism 20, comprises images of the die 4 and the substrate 10 superimposed upon each other. Image processing software can be used to determine the relative locations of the reference marks A, B relative to the reference marks Axe2x80x2, Bxe2x80x2 respectively, and to signal an associated control system to move the die carrier 6 and/or the substrate carrier 16 until the reference marks A, B are accurately aligned with the reference marks Axe2x80x2, Bxe2x80x2, respectively.
The alignment method described above has various disadvantages and shortcomings. For example, with reference to FIG. 1, portions 40a, 40b of the light beams 36a, 36b (i.e., the image of the die 4) pass through the interface 34, project an image of die 4 onto the substrate 10, and are then reflected back to the beam splitter 2 from the substrate 10. Similarly, portions 45a, 45b of the light beams 41a, 41b (i.e., the image of the substrate 10) pass through the interface 34, project an image of the substrate 10 onto the die 4, and are then reflected back to the beam-splitter 2 from the die 4. These reflected images create interference fringes or blurring of the image received by the image processor 3. Such effects can increase the difficulty of accurately aligning the die 4, and the substrate 10 with each other.
One approach to overcoming this problem is to generate separate images of the die 4 and substrate 10, and combine the images digitally. For example, if the interface 34 were made to be 100% reflective, the image processor 3 would receive only the image of the die 4 at the face 30 of the prism 20. A second image processor would be provided to capture the image of the substrate 10 at the face 24 of the prism 18 (which is made to be transparent), and the two images would be superimposed by digital manipulation (see, e.g., U.S. Pat. No. 4,899,921 to Bendat, et al.). Besides the increased cost of equipment to capture and combine two images, it would be necessary to carefully calibrate the image processors to accurately track the positions of the die carrier 6 and the substrate carrier 16 relative to each other.
The present invention overcomes the disadvantages and shortcomings of the prior art discussed above by providing a new and improved optical device adapted to superimpose the image of a die positioned at one side of the device and the image of a substrate positioned at an opposite side of the device. In one embodiment, the device comprises a plurality of reflective surfaces arranged so that the superposition of images takes place at a partially reflective surface within the probe. The superimposed image is displaced laterally from the die and the substrate. Neither the superimposed image nor the individual images of the die or the substrate is projected onto either component. Preferably, the optical device comprises a pair of right triangular prisms, each having a mirror hypotenuse face, a pentaprism having a pair of opposed inclined mirror faces, and an optical cube beam-splitter comprising the partially reflective surface.
In another embodiment, the optical device is a component of a single-camera optical probe for use in aligning the die with the substrate. The camera receives the superimposed image of the die and the substrate together that is produced at the partially reflective interface, and converts the image to a digital signal. The image received by the camera can, thereby, be monitored by image recognition software or by an operator to observe and correct the alignment of the die and the substrate.